This invention relates to a method and apparatus for measuring the distribution of light field about the optical axis of a light source such as a light projector and a motor car headlight, and more particularly to a method and apparatus for measuring the distribution of light field about the optical axis capable of readily adjusting the optical axis of a headlight of a motor car with high accuracies.
As will be described later with reference to the accompanying drawings, a prior art method of testing and adjusting the optical axis of a motor car headlight, for example, comprises the steps of disposing light receiving elements at the center and on the circumference of a circle drawn on a flat plate at an angular spacing of 90.degree., aligning a light emitting portion of the headlight which is disposed to face the flat plate with the center of the circle, illuminating the flat plate with a light beam from the headlight, coarsely adjusting the vertical projection angle and the horizontal projection angle of the light beam such that the respective light receiving elements will receive the same amount of the incident light, and then finely adjusting the vertical projection angle and the horizontal projection angle such that the light receiving element at the center of the circle will receive the maximum amount of the incident light. With this method, however, the distribution of the amount of the incident light upon the flat plate from the headlight should be symmetrical with respect to the center of the circle not only in the vertical direction but also in the horizontal direction. If the distribution of the amount of the light were asymmetrical with respect to the center, measurment errors would result.
Also, as will be described later, another prior art method of testing and adjusting the optical axis of a headlight comprises the steps of projecting the light of the headlight upon an irregularly reflecting screen, picking up an image of the projected light on the screen by means of a video camera, processing the output video signal from the video camera so as to produce pulses at the transition point of maximum brightness on the screen and the transition points on respective equibrightness curve, thereby to display border lines on a video monitor. According to this method, the light distribution on the screen can be displayed continuously if the tone of a monitor picture varies within a short time in the horizontal direction, but it is impossible to judge the continuity of the equibrightness curve if the time of variation in the brightness is long in the horizontal direction.